Forming joins between sheets of metal which are both strong and of sufficient aesthetic appeal is an ongoing issue for many industries, in particular the automotive industry. One area of joining where significant problems remain is when making a join between sheets of two different metals.
One technique which is widely used in joining sheets of metal in high volume production is spot welding or butt welding, which may or may not involve the use of a laser. Laser welding is particularly suitable for joining sheets of different thicknesses and results in neat and narrow joins, produced at high speed. However, a number of problems exist in laser welding. One such problem is that when using such a technique to join magnesium to aluminium alloys (as with any technique which involves at least some melting), the laser welding results in the formation of hard and brittle inter-metallic phases around the weld. Furthermore, such techniques are not particularly suitable to magnesium alloys formed by a high pressure die casting process because of the trapped gases in such castings.
This problem is not specific to magnesium when using laser welding. In the case of steel, for example, the formation of hard phases such as martensite about the laser welded join impairs the ductility of the stainless or non-stainless steel. In the case of joins between aluminium/steel elements, inter-metallic Fe—Al phases grow to form a layer at the interface. To produce Fe—Al joints of acceptable mechanical properties, the thickness of this inter-metallic layer must be less than 10 μm and therefore requires appropriate temperature-time cycles.
When laser welding, at least one of the components is melted. Laser welding of aluminium to steel is like a brazing process, where only the lower melting point alloy undergoes fusion. On the other hand, laser welding of magnesium and aluminium leads to the melting of both metals so that the inter-diffusion of chemical elements cannot be prevented. This results in the formation of inter-metallic phases such as for example β-Mg17(Al, Zn)12, which has to be minimised in order to form a satisfactory join.
An alternative technique for forming a join is friction stir welding (FSW) which is more tolerant to dissimilar metals being joined and to high pressure die castings (which have trapped gases) because it is a solid state joining process, whereby two materials are plasticised under the action of a probe. Thus, the metals are not melted when friction stir welded. A number of problems, however, exist with friction stir welding, including the low speed of the process, the limited part geometry to which the process can be applied and the high force requirements, which may require a high amount of power and special tooling for friction and clamping. Furthermore, evidence exists to suggest that when using friction stir welding to join magnesium to aluminium, Al3Mg2 and Al12Mg17 phases are formed about the weld which may crack under stress.
As an alternative to the above methods, cold joining technologies may be employed to join dissimilar metals without the formation of undesirable metallurgical compounds and to avoid the issues relating to trapped gases in high pressure die castings. One such technique is bolting, which has a number of its own problems, including the added weight, the requirement for accurate alignment, fretting and wear between the bolt and the metal part (particularly where the metal is magnesium or magnesium alloy), and increased material costs in providing the bolts.
An alternative cold joining technique is that of self-piercing riveting (SPR) which is particularly suitable for joining dissimilar metals which are reasonably formable. An advantage of self-piercing riveting is that galvanic corrosion can be minimised. However, when joining aluminium and magnesium alloys using an SPR process for example, a significant problem arises when the magnesium is on the bottom (die) side of the join, where the high tensile strains deforming the magnesium, tend to cause the magnesium to crack because of its poor ductility at room temperature.
Attempts have been made to overcome this problem by preheating the magnesium. An example of this is described in DE 19630488 in which induction heating is used to preheat the magnesium in forming the SPR joint. However, a number of problems exist with this process, including high power requirements, long cycle times and difficulties inherent with the tooling, which is bulky and difficult to accommodate. These problems reduce the economic viability of the technique.